The subject matter of this invention relates generally to gating circuits for high voltage series connected thyristor strings, and more particularly, to gating circuits utilizing current transformers as energy sources.
It is well known to use gate controlled thyristors to control the conduction interval in a conductor connected between two voltage potentials. During a conduction interval, the thyristor switch is gated on and acts as a conductor path connecting the two sources of electrical potential, one with the other, usually through a suitable impedance for current limiting purposes. During a controlled period of nonconduction, the thyristor acts as an open circuit and therefore is subjected to the voltage difference between the two voltage potentials. In the latter situation, the physical characteristic of the thyristor must be such as to resist voltage breakdown. Every thyristor device has a maximum voltage blocking figure beyond which voltage breakdown will occur. In high voltage situations, the aforementioned voltage figure may be insufficient if only one thyristor is utilized. Consequently, a series string of synchronously controlled thyristors is used, each of which is proportionately subjected to reduced breakdown voltage. In the foregoing situation, each of the thyristors usually easily accommodates its proportional share of blocking voltage; however, close scrutiny will indicate that another voltage problem may exist in some instances. In particular, the cathode of some or all of the thyristors in the string of thyristors may be at high potentials, even though the net voltage across the thyristor is within the voltage blocking capability of the thyristor. Since thyristors are controllable devices, energy must be made available at the cathode potential of the thyristor for causing a gating action in the thyristor at an appropriate time and under appropriate circumstances. It is to be noted that the amount of energy per gating action is usually small, in the order of 10 millijoules; however, the voltage potential at which this energy is utilized may be relatively high. Consequently, one who is utilizing high voltage thyristor strings is faced with the problem of delivering the relatively small amount of energy to control a thyristor whose cathode is at a relatively high voltage potential. In the past, a number of solutions have been provided to solve the aforementioned problem. One solution is to produce the required energy at ground potential and then feed the energy through cascaded transformers to the high potential of the cathode of the thyristor. This is sometimes known as a "magnetic" solution (because of the reliance on transformers). It has the disadvantage of requiring the utilization of numerous transformers which lead to weight, volume, size, cost and manufacturing problems because of the insulation that is necessary to isolate the high cathode potential at which the energy is utilized from ground potential at which the energy is produced. A solution to the above-mentioned problem is to derive the energy at the cathode potential of the thyristor. This is done by means of a voltage divider network which is usually added to the R-C snubber circuit of the thyristor. The voltage divider network usually comprises a suitably sized capacitor, one electrode of which is connected to the thyristor cathode and the other of which is connected to a portion of the snubber network. The capacitor charges as voltage is developed across it when the thyristor is in a nonconducting stage; however, it is to be realized that voltage must be developed across the capacitor to provide the energy which is later utilized to fire the gate of the thyristor. The voltage may come from the terminals of the thyristor in question or from another thyristor, but voltage of the proper polarity must be present in order to charge the capacitor. This is sometimes known as the "voltage divider solution". Although there are many areas in which the above-mentioned problem may be significant, two interesting areas are associated with converter bridges and VAR generators. A converter bridge application of thyristors is taught in U.S. Pat. No. 3,386,027, issued May 28, 1968 to L. A. Kilgore et al and entitled, "High Voltage Converter Apparatus Having A Plurality Of Serially Connected Controllable Semiconductor Devices" (assigned to the assignee of the present invention). It is not necessary at this time to explain the dynamics of the operation of a converter as the aforementioned patent describes that in detail. It is sufficient for purposes of this invention to realize that in a converter situation (or for AC control of resistors) the voltage across any thyristor may vary over a wide range, and may even be zero. This depends on the momentary operating conditions of the system. Obviously, in such systems, the previously mentioned "voltage divider approach" is ineffective. Another area in which the string of thyristors is used is the VAR generator situation. A typical VAR generator is disclosed in U.S. Pat. No. 3,999,117, issued Dec. 21, 1976 to Gyugyi et al and entitled, "Method And Control Apparatus For Static VAR Generator And Compensator" (assigned to the assignee of the present invention). Once again, the description of the operation of a VAR generator is not critically important for this application as it is well described in the latter-mentioned patent. In the latter situation, overvoltage for each thyristor can become a significant problem. It will be remembered that each thyristor is generally sized so that it can accommodate a certain predetermined amount of blocking voltage; however, in some instances which may be associated with load shedding, lightning or other transient phenomena for instance, the voltage impressed across the string of thyristors may be so high as to jeopardize the voltage blocking limits of each thyristor. In such a case, one of two things can be done. The overvoltage can be anticipated in the design of the thyristor string, thus leading to conservative blocking voltage levels. This obviously works, but creates the undesirable problem of unnecessary blocking voltage capacity when most of the time a non-overvoltage situation will exist. This tends to be an inefficient use of the thyristors. Another solution is to merely deliberately cause all thyristors to conduct during a sensed overvoltage situation. The well known reactance which is connected in series with the various thyristors in the VAR generator situation acts to limit any current which may flow, while the conducting thyristors are not faced with an undesirable voltage blocking situation. In order to do this, voltage must be present. It may be that the thyristor network must accommodate overvoltage for longer than one-half cycle, in which case, subsequent firings of the thyristors will be jeopardized because of the absence of charging voltage during a previous half-cycle. Regardless of whether the converter situation or the VAR generator situation is examined, it becomes apparent that a situation can arise in which no energy is available for firing the thyristors because of the lack of charging voltage during a previous interval. Corollary to this is the realization that in those cases in which no voltage is present for charging the capacitor, there is likely to be significant current conducted by the thyristor. It would be advantageous, therefore, if a way could be found to utilize the conducting aspects of the thyristor, i.e., its current during a conduction interval, to provide energy during an initial portion of a half-cycle, for example, or to provide energy during other half cycles which energy ultimately is utilized to charge a capacitor or other wise deliver energy to the gate of the thyristor or silicon controlled rectifier for actuating the gate to cause conduction.